For the configuration shown in the figure below, suppose a = 5.00 cm, b = 20.0 cm, and c = 25.0 cm. Furthermore, suppose the electric field at a point 10.5 cm from the center is measured to be 3.25 × 10^3 N/C radially inward and the electric field at a point 50.0 cm from the center is of magnitude 176 N/C and points radially outward. From this information, find the following. (Include the sign of the charge in your answer.) (a) the charge on the insulating sphere i have calculated as follow: E( the surface area of sphere) = q(inside)/ permittivity 3.25χ10^3 (4χ3.16χ0.105^2)= q(inside)/ (8.85χ10^-12) q(inside) = -0.000000004 C this is the correct answer but i don't know why my equation is valid. First, there is charge in the insulator and the inner wall of the conductor, right? and isn't the electric field measured at 10.5 cm from the center of the insulator the sum of the effect from both the charge from the insulator and the inner wall of the conductor? if it is true, then why i can use the gauss law to find the total charge inside the insulator like this?